This invention relates to an apparatus for delivering two or more liquids under precise proportioning or fluid ratio control. More specifically, the apparatus relates to a reciprocating pumping system having variable stroke metering cylinders which are interconnected so as to form a unitary liquid delivery system wherein the proportions of each liquid may be precisely regulated.
The need for fluid ratio delivery systems is increasing as more and more fluids are developed which may be combined at some ultimate mixing or destination point to form a mixture suitable for particular applications. For example, some modern paint compositions require the precise mixing of a base or catalyst material with a colorant additive, the combined mixture of which is delivered to a paint application system. It is frequently necessary to isolate the components prior to mixing, as they have certain volatile curing characteristics which give them a very short useful life when mixed, but which enables them to be stored over rather long periods of time in an unmixed condition. For this type of material it is inadvisable to premix the components at the point of manufacture because they would either require special sealing and handling conditions prior to their use or they would become severely degraded in quality and composition. In instances where a single base material may be mixed with any of a plurality of optional color components it is economically more desirable to package the base material in a separate container and to package each of the components in individual containers.
Precision proportioning of liquids using reciprocating pumping devices presents particular problems, because fluid metering becomes unpredictable and somewhat erratic at the point where the reciprocating device changes its stroke direction. On the other hand, reciprocating pumping devices have certain advantages in that they are simply and economically constructed and may be driven readily by reciprocating air or hydraulic motors, or other pressurized fluid sources. It is possible to minimize the changeover metering problem by ganging together a plurality of these metering cylinders so that all cylinders go through their changeover stroke phase at the same instant. This does not eliminate the unstable metering situation which exists during changeover, but tends to equalize the instability by requiring all metering cylinders to undergo change-over together. Thus, if fluid delivery from a particular metering cylinder is reduced by a significant percentage during the short interval of time while the piston changes its reciprocating direction, other cylinders having pistons ganged together will similarily reduce their flows by nearly identical percentages during the same interval of time. The net result may be an overall fluctuation in the flow rate of the combined mixture, but the relative proportions of each of the components of the mixture will tend to vary in the same direction and ensure that the flow ratios remain constant. These desirable effects are achieved with the present invention, which uses reciprocating metering cylinders to the advantages stated herein.